Infectious Tree Diseases - Why Trees Matter

Infectious
Tree Diseases
The more you know about tree health, the better you’ll
be able to manage disease incidents. This primer will
give you a leg up on the competition, whether human
or pathogen.
B Y J I M C H AT F I E L D , D E N I S E
E L L S WO R T H A N D J O E B O G G S
12
A M E R I C A N N U R S E RY M A N
F E B RUA RY 2 0 1 1
AN_Feb11_1-15.indd 12
PHOTOS COURTESY OF JIM CHATFIELD
I
nfectious tree diseases—those
diseases that involve not only a tree
host, but also an infectious pathogen
such as certain fungi, bacteria, nematodes, phytoplasmas or viruses—are
among the most vexing maladies that
we deal with as horticulturists. Let’s
take a look at why this is the case. First,
of what do we speak?
Dutch elm disease. Verticillium wilt
on maple and smoketree and tuliptree
and many other hosts. Oak wilt. Fireblight on pear, mountainash and other
members of the rose family (Rosaceae).
Plum black knot on plums, cherries,
peaches, almonds and their ornamental cousins. Pine wilt disease. Phytophthora root and crown rot of beech and
other hosts. Apple scab on crabapple.
Tar spot of maple. Anthracnose on
sycamore, anthracnose on dogwood
and anthracnose on oak. Bacterial leaf
scorch. Sudden oak death. Thousand
cankers disease. Powdery mildews on
many different trees.
What do all of the above have in
common? They are all infectious tree
diseases. Some are vascular diseases,
destructive because the pathogen
moves throughout the plant in the vascular, or conducting, system. These include Dutch elm disease, Verticillium
wilt and oak wilt. Some are leaf diseases, such as apple scab, or leaf and twig
Guignardia leaf blight is evident on Aesculus. With such diseases, there is no control
after infection strikes and symptoms appear. Above right, ruiting bodies of the Diplodia fungus, seen here, produce microscopic spores that infect tissue in spring.
diseases, such as anthracnose diseases.
Some are worse than others, such as
dogwood anthracnose, which is more
severe than oak anthracnose.
Some are caused by pathogens
with narrow host ranges, such as Dibotryon morbosum that causes plum
black knot only on the genus Prunus.
Some pathogens have wide plant host
ranges, such as the Verticillium fungus
with scores of susceptible tree host
genera. Some of the pathogens infect
through roots, such as the Verticillium
fungus, while others infect through
stem and leaf tissue, such as the anthracnose fungi.
Some pathogens are moved from
tree to tree through insect vectors, such
as the Ophiostoma ulmi fungal pathogen that causes Dutch elm disease,
vectored by elm bark beetles. Most infectious diseases are caused by fungi,
while others are caused by bacteria
(fireblight, for example) and nematodes, such as pine wilt disease caused
by pine wood nematode. Some diseases, such as Dutch elm disease, are
devastating to plant health, while others cause mostly ornamental damage,
such as powdery mildews and tar spot
of maple.
All these plant diseases are fascinating and important to green industry professionals as they select, install,
maintain, diagnose and treat trees.
There are features of plant diseases,
though, that confound and frustrate
not only professionals, but also our
customers and clientele. Here are a few
things you should know about infectious tree diseases.
Why are infectious plant diseases
so difficult to manage? Let us count just
a few of the ways.
www.amerinursery.com
1/26/11 10:52:52 AM
• Inoculum is microscopic
• Inoculation, infection, symptoms are separated in time
• Disease control is largely preventive, not reactive
• Weather is unpredictable
• Pathogens change
• There are so many host plants
Sycamore anthracnose is worse on American planetree (sycamore on left) than the
hybrid London planetree (right).
Inoculum is microscopic
Inoculum, what Gail Schumann
and Cleora D’Arcy define in their excellent book Essential Plant Pathology is
“the structure or part of the pathogen
that initiates disease.” Inoculum can be
microscopic spores or thread-like mycelia of fungi, microscopic bacterial cells,
microscopic parasitic eelworms known
as plant parasitic nematodes, or submicroscopic particles of viruses or phytoplasmas. So small, even regular light microscopes cannot detect this inoculum.
One of the reasons that infectious
plant diseases were mysterious for so
long, and why disease management is
complicated, is that the inoculum of
the pathogen is so small that it is invisible to the naked eye when it arrives at
the plant, penetrates plant tissue and
infects plant tissue. To use a non-tree
example, when late blight of potato
threatened crops and helped lead to
the Irish Potato Famine, the venerable Gardener’s Chronicle opined, “It
was suggested that the rot might be
caused by static electricity generated in
the atmosphere by the issuing puffs of
smoke and steam from the hundreds of
railway locomotives that had recently
come into use, and for all that was surely known it might equally well be due,
as others supposed, to mortiferous
vapors or ‘miasmas’ rising from blind
volcanoes in the interior of the earth.”
We know better now. We have a
germ theory of disease. But, let’s not be
too proud of our superior knowledge:
The invisibility of pathogens to the naked eye when they arrive, penetrate and
cause infections on plants still boggles
the minds of many if not most of us. We
Continued on page 14
For free information circle 19 on the fax-back form on page 37
Willamette_12H_1210 1
www.amerinursery.com
AN_Feb11_1-15.indd 13
A M E R I C A N N11/29/10
U R S E RY9:16:03
M A N AM
13
F E B RUA RY 2 0 1 1
1/26/11 10:53:26 AM
Infectious Tree Diseases
Continued from page 13
consider insects and mites to be small
and hard to see, but they are gargantuan
compared to “virulent plant pathogens”
that cause infectious plant disease,
along with the other two components
of the disease triangle: the “susceptible
host” and the “environment conducive
to disease.”
Powdery mildew on ninebark can cause considerable aesthetic damage.
Dogwood anthracnose is more serious than
other anthracnose diseases. Each disease is
unique.
Inoculation, infection,
symptoms separated in time
So, the inoculum that arrives at
the plant via winds, splashing rain or
a vector cannot be seen. Then a spore
germinates and penetrates into plant
tissue, or a nematode inserts its stylus
into the plant, or a vector inserts the
pathogen into plant tissue; you still
cannot see the pathogen. The pathogen then spreads through the plant
and establishes a host-parasite relationship with plant cells.
You still cannot see the pathogen.
In fact, you have no idea until symptoms
develop on the plant, for example, leaf
blighting and discoloration along the
veins of a sycamore due to an infection
from the sycamore anthracnose pathogen. Symptoms develop for days, usually
a week or more, and sometimes months
or years after inoculation, penetration
and infection. As tree care professionals, we are effectively in the dark all that
time. Among other things, this makes
effective timing and use of disease-controlling pesticides, such as fungicides,
difficult. Symptom development and
damage to the plant may be inevitable,
even when the infected plant looks fine.
Disease control is
preventive, not reactive
So, we do not exactly live in a
cloud of ignorance, in a world of “blind
14
A M E R I C A N N U R S E RY M A N
F E B RUA RY 2 0 1 1
AN_Feb11_1-15.indd 14
volcanoes” and “mortiferous vapors.”
Nevertheless, since we cannot see inoculation and infections occur, we must act
proactively with regard to plant diseases.
This means that in many cases we need
to use preventive fungicide applications
to prevent that germinated fungal spore
from getting into plant tissue in the first
place (although there are systemic fungicides that help in some cases).
Better yet and more sustainably,
we need to employ our knowledge of
genetic disease resistance, plant health
and plant stress management to optimal effect. We need to recognize that
‘Sargent’ and ‘Adirondack’ crabapple
have excellent resistance to apple scab,
compared to ‘Thunderchild’ and ‘Hopa’.
If we know that we have a site with the
Verticillium fungus well established in
the soil, which we can know from previous Verticillium wilt diagnoses, then do
not plant highly Verticillium-susceptible species, such as Japanese maples,
(Acer palmatum) into that site.
Selecting the right plant for the
right site (“which plants matter”) is the
greatest preventive maintenance practice we can use, not only for infectious
disease management, but also for overall plant health. Eastern redbuds that
are planted in open sun, unirrigated
sites are more likely to become moisture stressed and are more likely to
have Verticillium wilt problems as well
as Botryosphaeria canker disease, the
two most common infectious disease
problems of Eastern redbuds that cause
stem dieback and eventual plant death.
Prevent from the very beginning by not
planting redbuds in those sites.
Weather is unpredictable
The environmental component of
the disease triangle is crucial for plant
diseases. Plant pathologists talk about
infection periods for specific plant
diseases, and whether or not these infection periods occur depends upon
such things as the number of hours of
leaf wetness and relative humidity. Of
course, this, in turn, is influenced by
temperature. All of this plays into the
overall weather, and we know how good
we are at predicting the weather, even
short term, much less long term, as in
over an entire growing season.
Temperature and moisture, for
example, play a big role in development of fireblight disease. Bad years for
fireblight on Callery pear or crabapple
usually relate to how warm and wet it is
during bloom. The years in which there
are massive blossom infections (greatest occurrence during wet weather during extended periods over 62 degrees)
are years in which fireblight is worse.
So it depends on whether these conditions occurred during the bloom of a
particular Callery pear or crabapple in
a particular part of the state or region.
Anthracnose diseases on oak, sycamore and ash depend greatly on cool,
wet conditions during leaf emergence
on these trees. Oak anthracnose is really
noticeable on the lower canopy (less air
movement) only once in a while—the
years in which that oak (mostly in the
white oak group) has leaves emerging
www.amerinursery.com
1/26/11 10:53:48 AM
during extended cool, wet weather. Bad
years for rose black spot depend upon
the number of times there are a certain
number of hours of leaf wetness (exact number of hours needed depends
upon the temperature variable) for the
pathogen spores to germinate.
For the most part, we cannot use
phenological calendars for infectious
tree diseases. Phenology is the relationship of environmental factors, such as
temperature, to physiological events,
such as flowering of plants or emergence
and development of insects; using it for
insect control timing is fairly straightforward, since it involves simply the temperature variable (heat units and degree
days). With plant diseases, however, it
is more complex because of the role of
moisture in addition to temperature.
set of diseases and other problems. Remember that the multiplicity of host
plants creates an extra layer of information for us to keep track of: apple scab
does not occur on roses; Dutch elm disease does not occur on oak; black knot
does not occur on pears; sycamore anthracnose does not occur on oak. Each
host not only has its own set of horticultural best practices, but also its own
set of disease weaknesses. Infectious
plant diseases are indeed a wonder.
Pathological puzzlers provide a neverending quest (don’t even pretend to
be a know-it-all) and plenty to wonder
about. As William Shakespeare noted:
“In nature’s infinite book of secrecy/A
little I can read.”
The authors are members of The Ohio
State University Extension Nursery
Landscape and Turf Team. Jim Chatfield
is an Extension specialist in the Department of Plant Pathology and the Department of Horticulture and Crop Science in Wooster, Ohio (chatfield.1@osu.
edu). Denise Ellsworth is an Extension
educator ([email protected]) in Akron. Joe Boggs is in the Department of
Entomology and an Extension educator
([email protected]) in Cincinnati.
Pathogens change
‘Indian Magic’ has excellent scab
resistance! Does not! Does too; I have
an old report to prove it! Did once; but
no longer!
The reality is that we are all proud
when we notice genetic resistance to
infectious diseases in plants. In fact,
sometimes we go to considerable
lengths and time with plant breeding
to develop plants that provide both desired arboricultural characteristics and
resistance to certain diseases. However,
we have to remember that Nature is
at it 24/7 in its own breeding experiments. Over time, pathogens mutate,
overcome resistance and parasitize
plants that once truly had good genetic
resistance relative to a particular disease. Such is life in the jungle. Disease
resistance is not necessarily forever, so
we have to keep studying and learning
and adapting.
There are so many host
plants
Physicians have many advantages
over plant doctors, though we at least
do not have to deal with mole removal,
skin diseases, brain surgery or the pressure of our patients dying a human
death. Physicians, however, have patients who talk, even if we so often lie.
Physicians also have the advantage of
dealing with only one host: Homo sapiens. As we know, even dealing with
the diseases and disorders of just one
species, humans, can be anything but
simple. As tree doctors, we deal with
hundreds of different host plants, different species of maples and oaks and
different genera from Acer to Zelkova,
from Quercus to Xanthophyllum.
Each of these plants has its own
ORDER ROSES NOW
for Late Spring and
Summer Delivery
Available in 36 & 58
cell plug trays:
UʏÕň˜}ʘœVŽÊ"ÕÌ® PP#14700
UÊ«ÀˆVœÌÊÀˆvÌ® PPAF
UʘœVŽÊ"ÕÌ® PP#11836
UÊ
œÀ>ÊÀˆvÌ® PP#19148
UÊ*ˆ˜ŽÊ˜œVŽÊ"ÕÌ® PP#15070
UÊVÞÊÀˆvÌ® PPAF
UÊ,>ˆ˜LœÜʘœVŽÊ"ÕÌ® PP#17346
UÊ*i>V…ÊÀˆvÌ® PP#18542
UÊ-՘˜ÞÊʘœVŽÊ"ÕÌ® PP#18562
UÊ*ˆ˜ŽÊÀˆvÌ® PP#18874
UʜÕLiʘœVŽÊ"ÕÌ® PP#16202
UÊ,i`ÊÀˆvÌ® PP#17877
UÊ*ˆ˜ŽÊœÕLiʘœVŽÊ"ÕÌ® PP#18507
UÊ-ÜiiÌÊÀˆvÌ® PPAF
Sold exclusively through
Star Roses and Syngenta
Horticultural Services.
For availability and contacts,
visit northwesthort.com.
800/459-3142
For free information circle 13 on the fax-back form on page 37
www.amerinursery.com
AN_Feb11_1-15.indd 15
A M E R I C A N N U R S E RY M A N
F E B RUA RY 2 0 1 1
15
1/26/11 10:53:54 AM